Ferromagnetism in minerals (e.g. magnetite) only persists below a critical temperature called the Curie point (~580°C for magnetite). Above this temperature, thermal agitation destroys the ordered magnetic domains, and the material becomes only weakly (paramagnetic) magnetic.
Earth's internal temperature rises steadily with depth, and this geothermal gradient means that below a certain depth in the crust (and definitely by the upper...
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